This invention relates generally to imaging systems and more particularly to, systems and methods for improving a resolution of an image.
An imaging system includes a source that emits signals including, but not limited to, x-ray, radio frequency, or sonar signals, and the signals are directed toward a subject, such as a patient, to be imaged. The emitted signals and the interposed subject interact to produce a response that is received by one or more detectors. The imaging system then processes the detected response signals to generate an image of the subject. For example, in computed tomography (CT) imaging, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the subject being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the subject to be imaged so that the angle at which the x-ray beam intersects the subject constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the subject includes a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the subject.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. The filtered backprojection technique converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a “helical” scan may be performed. To perform a “helical” scan, the subject is translated along a z-axis while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one-fan-beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
Typically, a CT scan is acquired at a single field of view, which accounts for a wide part of an anatomy of the subject. As a result, an entire volume including the wide part is imaged and viewed at the same resolution. In certain instances the resolution is lower than that needed for an anatomical region and hence the ability to resolve fine structures is compromised. For example, in head and neck cases, a CT angiogram display field-of-view (DFOV) parameter is set to accommodate the wide part of anatomy in the scan, which are the shoulders. This DFOV setting is used to reconstruct a volume of the head. However, proximal to the shoulders, the width of the neck is much smaller and the DFOV reconstruction reduces an in-plane resolution in the neck. The lower in-plane resolution in the neck is further compounded by a small size of a plurality of objects of interest, such as, a plurality of vertebral arteries. At a large DFOV setting, such as 34 centimeters (cm), a width of one of the vertebral arteries in a plurality of axial planes is less than five pixels. Anatomical analysis reveals that 10% of the normal population present a smaller right vertebral artery compared to a left vertebral artery that is ipsilateral to the heart. With the right vertebral artery running through a bright cortical bone of a cervical transverse process, a boundary of the right vertebral artery is buried in partial volume. At a plurality of critical locations, such as the boundary, separating a foreground object, such as one of the vertebral arteries that is 5 pixels wide, without excursion into a neighboring object, such as a bone, or background becomes difficult. Another anatomical region that presents a similar challenge is a plurality of peripherals, such as arms and legs, of the subject. The arms and legs are also scanned at a wide FOV corresponding to an abdomen of the subject. Peripheral arteries of the subject are typically less than 2-3 millimeters (mm) in diameter and span a few voxels after reconstruction.